Convection strip heater and control therefor



July 11, 1961 D. K. MARTIN 2,991,989

CONVECTION STRIP HEATER AND CONTROL THEREFOR Filed March 10, 1958 AIR sTEAM jg 2 ffyj PQOPORTIONING MlXER INVENTOR. D. K. MARTIN ATTOR N EYUnited States PatentOice 2,991,989 CONVECTION STRIP HEATER AND CONTROLTHEREFOR Donald K. Martin, Toledo, Ohio, assignor, by mesne assignments,to Midland-Ross Corporation, Cleveland, Ohio, a corporation of OhioFiled Mar. 10, 1958, Ser. No. 720,342 3 Claims. (Cl. 263-3) Thisinvention relates to heating of relatively thin material, and moreparticularly to heating metal such as steel, brass or aluminum in stripform by passing a strand thereof through a ductwhich constitutes aconvection heating zone, and circulating through the duct at highvelocity a stream of gas, preferably an atmosphere which will protectthe surface of the metal being heated, the gas being at a temperatureconsiderably in excess of the final desired work temperature.

Convection heating of metal strip by application there to of highvelocity streams of gas whose temperature is relatively high ishereafter referred to as high heat head convection heating of strip. Insuch heating very high heating rates are obtained, and the size ofequipment required is substantially smaller and, correspondingly,somewhat less expensive. At the same time unusually uniform treatment inthe strip is obtained. T

To avoid overheating of the strip in the event of strip stoppage, themass of the heater is substantially reduced from that of the usualfurnace structures, but it is desired also to rapidly reduce the movinggas temperature to safe temperatures. For example, if steel strip isheated to 1200 F. in a convection gas stream which is maintained at1600" F. it is desirable to rapidly reduce the gas tem perature to about1150 to 1200 F. to avoid locally overheating the steel strip.

For a further consideration of what I considerto be novel and myinvention, attention is directed to the drawing, the following portionof the specification and the claims appended thereto.

In the drawing:

FIG. 1 is a schematic representation of a convection heater according tothis invention.

FIG. 2. shows an alternate detail for the apparatus of FIG. 1.

FIG. 3 shows an alternate detail for the apparatus of FIG. 1.

FIG. 4 is a schematic diagram of a portion of the coni V trol system forthe apparatus of FIG. 1.

The convection heater of FIG. 1 comprises a heating duct 11 forming apart of a closed circuit for a convection heating stream of gas, inletand outlet chambers 12 and 13 for strip passing through the heatingduct, entrance and exit roll seals 14 and 15' to seal the atmosphere gasin the closed circuit from outside air, and a recirculating pipe 16comprising a blower 17 and a heater 18. Strip is drawn through theheating duct 11 by driving rolls 21 operated by a strip drive motor 22.Heating gas is circulated in the heating duct and through the closedcircuit by the blower, which preferably is a constantly operatingblower.

Where flue gas forms a suitable atmosphere for the metal to be heated,as may be the case in many copper base alloys, the heater may be adirect fired type of heater, and a vent, not shown, may be provided forexcess atmosphere gas. In many cases, as for example in heating steelstrip, an indirect heater will be used and a specially preparedatmosphere gas will be supplied to the closed circuit by an atmosphereinlet pipe, not shown, and flue gas from the heater will be vented fromvent 20.

The heater 18 receives a mixture of fuel and air from proportioningmixer 9 via mixture pipe 26. Fuel is supplied to proportioning mixer 9from fuel supply pipe Patented July 11, 1961 23 through fuel valve 8,and air is supplied to proportioning mixer 9 from air supply pipe 24through proportioning air valve 25, the proportioning air valve beingoperated by a proportioning air valve motor 27 responsive to'suitabletemperature control apparatus. The proportionin'gunit 9 may be aconventional proportioning mixer, not shown, such as is illustrated inUS. patent to Barber 1,914,905. The zero pressure gas regulator of suchmixers limits the flow of fuel from pipe 23 to a proportion of thevolume of air passed by valve 25.

When moving strip is heated in high heat head apparatus, an interruptionin the progress of the strip will necessarily result in overheating ofthe strip unless the heat head can be eliminated in sufficient time.Toward this end, the heating duct 11 is designed for a of heat storage,although it may be insulated. To provide for eliminating the heat headin the event of strip stoppage, cooling means are provided in the closedcircuit, preferably upstream of the blower. This cooling means may takethe form of an air inlet damper 28, as shown in FIG. 2, when the metalstrip being heated is not damaged thereby, as in heating aluminum strip,or it may take the form of a steam (or water) jet 31. as shown in FIG.3' for some copper base alloys not harmed thereby. For steel it may bean indirect cooler such as an indirect water cooled heat exchanger 32 asshown in FIG. 1, which may be constantly cooled to avoid overheating theheat exchanger. The heat exchanger 32 may be in a by pass duct as isillustrated inpatent to Cone 2,479,814 (FIGS. 2 and 3). It is alsopreferable to interrupt fuel flow to the heater 18 by closing mixturevalve 25 and pass only air therethrough by opening valve 44 until thecirculating gas stream drops to the desired temperature. In some cases,as when heating relatively thick strip, or metal less sensitive to shortover-heating, it may be sufficient to cool by passing air only throughthe heater, especially in a direct fired heater, and other cooling meansmay be dispensed with if such air can be supplied in sufficient volume.1 v

The controls for the apparatus of FIG. 1 include a Run temperaturecontrol instrument 30.for controlling the heater 18 responsive to a heatsensing device T0 shown as a thermocouple in the outlet chamber 13. TheRun control instrument 30. is of the proportioning control type and itswires ML, MC and MA pass from its slide-wire 33 through its control box34 to the respective contacts of a strip control switch 35 to the slidewire 36 ofthe proportioning air valve motor 27. When the switch 35 is inthe Run position and the strip is moving at normal speed, the Runtemperature control instrument 30 controls operation of the heater byadjusting valve 25 through motor 27 responsive to the heat sensingdevice TC the control setting being usually about 400 F. higher than thefinal desired temperature of the wonk. The strip switch 35 may be amanual switch which also controls the strip drive motor 22 throughcontacts 37, as shown in FIG. 4.

When the strip switch 35 is moved to its Hold position, the Run contactsbetween the motor 27 and the Run instrument 30 are opened and the wiresML, MC and MA are connected from the motor through Hold contacts in theswitch 35- to a Hold temperature control instrument 38, through itscontrol box 40 to its slide wire 41. The Hold instrument then controlsthe motor 27 to operate the heater 18 at a lower predeterminedtemperature at or below the final desired temperature of the work. TheHold instrument 38 controls the motor 27 responsive to a heat sensingdevice T0 in the inlet chamber '12. The location of sensing devices TCand TC is largely a matter of choice.

To rapidly cool the recirculating gas upon switching strip switch 35 toHold, a Hold contact 42 is made to close a circuit to an valve solenoid43 for a valve 44 in a by-pass air line around the proportioning controlvalve 25; hence when the controls call for reduced heat due to the lowersetting on Hold instrument 38 than maintained on Run instrument 30,contact 45 will be closed,contact 427' will be closed by switch 35, andthe valve 25 closes, an air inlet bypass valve 44 passes cooling throughthe heater to supplement the cooling effect of the cooling water coil 32in the inlet to the blower 17. The by-pass valve 44 is subject to acontact 45 in Hold control instrument 38-, so that as the desiredtemperature range is reached on cooling, instrument 3 8 goes on tonormal temperature control and the by-pass air valve 44 closes.

'It is, of course, within the scope of this invention to combine theelements of the control system-into a single instrument having alternatetemperature control ranges, or to operate both hold and run ranges froma single heat sensing element. shift from Run to Hold'may be responsiveto an. inertia switch 35 associated with a roller 21 on the strip, whichswitch in turn is operatively connected to the strip drive motor.

When this invention is applied to continuous strip heating the use ofexpensive looping pits and associated equipment may be dispensed with,since the strip will not be damaged by delays in the furnace. Thisapplication is a continuation-in-part of my application Serial No.475,121, filed December 14, 1954, now abandoned.

I claim:

1. A control system for a high heat head convection heater, whichcomprises the combination of va convection heating duct, through whichmetal strip material to be heated is passed; a conduit for circulatinggases from the outlet of the duct to the inlet of the duct; blower meansfor circulating gases through .the conduit and the duct; first andsecond gas temperature responsive means responsive to the temperature ofthe circulating gases; heating means for supplying heat to thecirculating gases;

valve means for varying the heating effect of the heating It is alsocontemplated that the material through the duct, and means responsive tooperation'of said drive motor for alternatively connecting said firstand second temperature control instruments to respond to saidtemperature responsive means.

2. A control system for a high heat head convection heater whichcomprises the combination of a'convection heating duct through whichmetal strip material to be heated is pased; a conduit for circulatinggases from the outlet of the duct to the inlet of the duct; blower meansfor circulating gases through the conduit and the duct; heating meansfor supplying heat to the circulating gases; valve means for varying theheating efiect of the heating means; a first temperature controlinstrument for adjusting said valve means in a manner to providecontinuous heating by the heating means to maintain relatively highcirculating gas temperature; a second temperature control instrument foradjusting said valve means in a manner to provide continuous heating bythe heating means to maintain a relatively low circulating gastemperature at or below the desired temperature for said material; and'means for alternatively operatively connecting said first and secondtemperature control instruments to said valve means.

3. Acontrol system for a high heat head convection heater whichcomprises the combination of a convection heating duct through whichmetal strip material to be heated is passed; a conduit for circulatinggases from the outlet of, the duct to the inlet of the duct; blowermeans for circulating gases through the conduit and the duct; heatingmeans for supplying heat to the circulating gases; heating valve meansfor controlling the effect of the heating means; cooling means forcooling the circulating gases; cooling valve means for varying theeffect of the cooling means; a first temperature control instrument foradjusting the heating valve means in a manner'to provide heating by theheating means to maintain a relatively high circulating gas temperature;a-second temperature control instrument for controlling the heatingvalve means and the cooling valve means to maintain a relatively lowerCirculating gas temperature; and selector means for alternativelyoperatively connecting the first and the second temperature controlinstruments to said valve means.

References Cited in the file of this patent UNITED STATES PATENTS I2,297,314 Offen Sept. 29, 1942 2,656,172 McGough Oct. 20, 1953 2,668,701Dietrich Feb. 9, 1954 1 FOREIGN PATENTS 948,545 France Jan. 31, 1949

